Most coal seams in China have characteristics including high gas pressure, high gas content, low permeability, and strong absorptivity, and it is very difficult to extract gasses from the coal seams. Therefore, it is an important approach to improve permeability manually for the coal seams to improve air permeability of the coal seams and improve the gas pre-extraction rate, in order to ensure safe production in the coal mines.
At present, hydraulic measures, represented by hydraulic slotting, etc., have been widely applied in the gas control process in the coal mining fields in China, owing to their efficient pressure relief and permeability improvement effect. However, owing to the fact that the geologic conditions of the coal seams in China are complicated and the permeability of the coal seams is low, if a single hydraulic measure is used solely, because of the limited fracturing ability of water-jet cutting and high-pressure water impact, the pressure relief and permeability improvement effect are limited, the gas extraction concentration will be low, the extraction cycle will be long, and the requirement for intensive coal mining can't be met.
In addition, available research findings have demonstrated that the gas absorptivity of a coal mass decreases by about 8% whenever the temperature increases by 1° C. In recent years, many researchers have put forward heat injection-based coal seam gas extraction techniques, which increase the temperature of a coal mass and thereby promote gas desorption by injecting high-temperature stream into a coal seam. However, the heat injection form is too simple, and the engineering application of these heat injection-based coal seam gas extraction techniques is rarely seen.